1. Field of the Invention
The present invention relates to a wavelength conversion element. More particularly, the invention relates to a wavelength conversion element for converting a wavelength of an optical beam into a shorter wavelength, using a nonlinear optical medium.
2. Description of the Related Background Art
A wavelength conversion element, specifically second harmonic generation (SHG) element, is an element for converting a wavelength .lambda. of laser into a half of .lambda. using an optical crystal material having a nonlinear effect.
Use of second harmonic allows a great increase in record density of information in an optical information record medium such as an optical disk. Therefore, recent development of SHG element becomes brisk these days. There are various SHG elements of three-dimensional waveguide known.
Conventional SHG elements are of a combination of high output coherent light source and bulk elements of nonlinear crystal. There is, however, a strong demand to make the optical disk apparatus and the laser printers smaller, so that semiconductor lasers have been replacing gas lasers as a light source recently. A light source of semiconductor laser has an output of several to several 10's mW, which requires the SHG element of thin film waveguide type to obtain a high conversion efficiency. Such SHG elements are disclosed for example in (1) Japanese Unexamined Patent Publication Sho 63-44781, (2) ELECTRONICS LETTERS Vol. 25 No. 3 (1989), and (3) 27a-p-10, Preview for the 51st academic presentations of Society of Applied Physics of Japan, Fall 1990.
In the SHG element as disclosed in (1), a substrate is formed of nonlinear optical crystal material of LiNbO.sub.3, and Ti is diffused on the substrate to form a waveguide. A diffraction grating is formed on the upper surface of the waveguide with a predetermined pitch. When a laser beam of wavelength .lambda. is introduced into an end of the waveguide from the laser light source as a fundamental wave, a second harmonic light (SHG light ) of a half of wavelength .lambda. is output from the other end of the waveguide in the thus-arranged SHG element. The diffraction grating provided on the upper surface of the waveguide serves to cancel a difference of refractive index between the fundamental wave laser beam and the SHG light. Thus the diffraction grating enhances a conversion efficiency into the SHG light by matching the phases of the fundamental wave laser beam and the SHG light. For this, the diffraction grating must have a pitch equal to a wavelength of the fundamental wave laser beam incident into the waveguide to satisfy the Bragg condition.
The phase matching may be conducted by index dispersion means for canceling the index difference between the fundamental wave and a higher harmonic by wavelength dispersion of nonlinear medium in the waveguide.
The above reference (2) discloses a domain inversion periodic structure as index dispersion means to generate the second harmonic with a high efficiency.
Also, the above reference (3) discloses an inversion region formed by thermal treatment of proton-exchanged LiTaO.sub.3.
Most of the conventional techniques use index dispersion means of domain inversion in the three-dimensional waveguide on LiNbO.sub.3 crystal to effect the phase matching. However, if the intensity of light in the waveguide is strong, the LiNbO.sub.3 crystal may suffer an optical damage where the refractive index varies, presenting a defect of mismatching in phase. Further, the LiNbO.sub.3 crystal has a relatively high Curie point, which leads to a difficulty in production of index dispersion means of domain inversion.
In addition, when the wavelength conversion element is applied to the optical disk apparatus or the laser printer to increase a capacity of information in the optical disk by a shorter wavelength light, it is essential for the element that the element is not against downsizing of the optical disk apparatus and that the intensity of shorter wavelength light can be modulated. The semiconductor laser is suitable for downsizing of the apparatus, but the gas laser is not. Although the semiconductor laser has a lower output as compared to the gas laser, the wavelength conversion element of waveguide type as described above may expect a high conversion efficiency even in use of the semiconductor laser as a laser light source.
However, it is difficult to output a modulated shorter wavelength light. Therefore, it is extremely difficult to obtain a wavelength conversion element suitable for the optical disk apparatus, which can output a shorter wavelength light with a high efficiency and which may modulate the shorter wavelength light. For example, if the semiconductor laser itself is modulated to modulate the shorter wavelength light, the wavelength of fundamental wave light would be unstable, which cannot supply a shorter wavelength light with a high efficiency.
As explained, the modulation of second harmonic itself is necessary for processing information or for recording information using the second harmonic. It would be convenient if a single element could perform both generation and modulation of second harmonic. There has been no such element obtained.